Yousef, B and Angus, DAC (2016) When do fractured media become seismically anisotropic? Some implications on quantifying fracture properties. Earth and Planetary Science Letters, 444. pp. 150-159. ISSN 0012-821X
Abstract
Fractures are pervasive features within the Earth's crust and they have a significant influence on the multi-physical response of the subsurface. The presence of coherent fracture sets often leads to observable seismic anisotropy enabling seismic techniques to remotely locate and characterise fracture systems. In this study, we confirm the general scale-dependence of seismic anisotropy and provide new results specific to shear-wave splitting (SWS). We find that SWS develops under conditions when the ratio of wavelength to fracture size (λS/d) is greater than 3, where Rayleigh scattering from coherent fractures leads to an effective anisotropy such that effective medium model (EMM) theory is qualitatively valid. When 1<λS/d<3 there is a transition from Rayleigh to Mie scattering, where no effective anisotropy develops and hence the SWS measurements are unstable. When λS/d<1 we observe geometric scattering and begin to see behaviour similar to transverse isotropy. We find that seismic anisotropy is more sensitive to fracture density than fracture compliance ratio. More importantly, we observe that the transition from scattering to an effective anisotropic regime occurs over a propagation distance between 1 and 2 wavelengths depending on the fracture density and compliance ratio. The existence of a transition zone means that inversion of seismic anisotropy parameters based on EMM will be fundamentally biased. More importantly, we observe that linear slip EMM commonly used in inverting fracture properties is inconsistent with our results and leads to errors of approximately 400% in fracture spacing (equivalent to fracture density) and 60% in fracture compliance. Although EMM representations can yield reliable estimates of fracture orientation and spatial location, our results show that EMM representations will systematically fail in providing quantitatively accurate estimates of other physical fracture properties, such as fracture density and compliance. Thus more robust and accurate quantitative estimates of in situ fracture properties will require improvements to effective medium models as well as the incorporation of full-waveform inversion techniques.
Metadata
Item Type: | Article |
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Authors/Creators: |
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Copyright, Publisher and Additional Information: | © 2016, Elsevier B.V. This is an author produced version of a paper published in Earth and Planetary Science Letters. Uploaded in accordance with the publisher's self-archiving policy. |
Keywords: | explicit fractures; finite-difference; full-waveform synthetics; scattering; seismic anisotropy; shear-wave splitting |
Dates: |
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Institution: | The University of Leeds |
Academic Units: | The University of Leeds > Faculty of Environment (Leeds) > School of Earth and Environment (Leeds) > Institute for Applied Geosciences (IAG) (Leeds) |
Funding Information: | Funder Grant number EPSRC EP/K021869/1 EPSRC EP/K035878/1 NERC NE/L000423/1 EPSRC EP/E500080/1 EPSRC EP/K035878/1 |
Depositing User: | Symplectic Publications |
Date Deposited: | 25 May 2016 12:18 |
Last Modified: | 04 Nov 2017 17:29 |
Published Version: | http://dx.doi.org/10.1016/j.epsl.2016.03.040 |
Status: | Published |
Publisher: | Elsevier |
Identification Number: | 10.1016/j.epsl.2016.03.040 |
Open Archives Initiative ID (OAI ID): | oai:eprints.whiterose.ac.uk:97216 |